The amnion is a unique membrane, composed of a single layer of cells, which completely line the cavity in which the foetus grows and develops. It is formed from the same small group of cells which are formed when the ovum fertilised by the sperm begins to divide. These cells have the potential to develop into all the cells of the human body and under the action of growth factors, and by a process which is as yet undefined, the cells will differentiate into either muscles, bones, heart, liver or whatever structure is required. We now have reason to believe that the cells which go to form the amnion retain this pluripotential characteristic of being able to differentiate, at least in part, into other tissues.
Another unique feature of the amniotic membrane is the complete lack of expression of surface antigens responsible for mounting an immune reaction. Thus, the amnion does not induce an immune response when transplanted into a "foreign" site, a feature which is of major importance to the foetus.
The body's immune system rapidly recognises foreign material and sets up an immune response which rapidly destroys the alien cells or organisms. Such considerations suggest that the fertilised ovum should be recognized by the body as a foreign or "non-self" tissue and be destroyed by the immune system. In fact, the ovum, and later the developing foetus, is spared from the immune response and is able to grow and flourish within the maternal uterus completely protected from the scavenging lymphocytes of the mother's immune system. Although the mechanism is not completely understood, the amnion is the key to the foetal protection. It is the immunological barrier between the mother as the host and the embryo as the "foreign" tissue.
While still in the fallopian tube, the fertilised ovum divides repeatedly to form a round mass of cells, the morula, from which develops both the foetus, the placenta and the amnion. It is this central origin of the amnion and its close relationship with the developing embryo that makes the amnion such a fascinating subject for research. An eccentric space appears in the morula resulting in a hollow sphere or blastocyst with a mount of cells on one aspect of the inner surface--the inner cell mass. The outer shell of the blastocyst becomes the trophoblast responsible for nutrition of the embryo (see FIG. 1).
The inner cell mass differentiates and forms two distinct masses, the outer (ectodermal) layer and the inner (endodermal). A further differentiation produces a third layer, the mesoderm, between these two (see FIG. 2). The combination of trophoblast and primitive mesoderm is termed the chorion (see FIG. 3). Two small cavities appear, one in the ectoderm forming the amniotic sac, the other in the endoderm--the yolk sac.
About the twelfth day after fertilization, the amniotic cavity is formed and the embryonic ectoderm of the inner cell mass takes shape from the floor of the amniotic cavity (Hertig 1968), the roof of which is formed by amniotic epithelium (Hertig 1945, Bourne 1942).
As the amniotic cavity continues to expand the amnion reaches the wall of the blastocyst. At the same time it involves the yolk sac. Part of the yolk sac becomes enclosed within the embryo while the remainder forms a vestigial tube which is applied to the original mesodermal stalk.
Blood vessels develop in the embryonic mesoderm and in the mesoderm of the trophoblast. Extension of these vessels along the connecting stalk result in the formation of the umbilical arteries and vein (see FIG. 4).
Within the embryo the vessel at the cephalic end differentiates to form the heart. Foetal blood formation occurs within the primitive blood vessels of the trophoblast and foetus. Interchange between mother and foetus is facilitated by the formation of this foeto-trophoblastic circulation. The formation and differentiation of the hemopoietic vascular system occurs between the third and fourth week of pregnancy. From then on full development of the foetus can take place.
Oxygenated blood from the placenta returns to the foetus via the umbilical vein. This vessel penetrates the liver and gives off small branches to that organ. Most of the blood is directed via the ductus venosus into the inferior vena cava which is carrying the returning non-oxygenated blood from the lower limbs, kidneys, liver and other organs. There is only partial mixing of the two blood streams and most of the oxygenated blood is directed by the crista dividens at the upper end of the inferior vena cava through the foramen ovale into the left atrium and thence to the left ventricle and aorta. This relatively well oxygenated blood supplies the head and upper extremities. The remainder of the blood from the superior vena cava mixes with that of the inferior cava, passes to the right ventricle and thence to the pulmonary arteries. A very small amount of blood actually perfuses the lungs. Most of it passes on via the ductus arteriosus into the aorta beyond the vessels supplying the head and other extremities. Thereafter it passes down the aorta to supply the viscera and lower limbs. Little blood actually perfuses the lower limbs. Most at this level passes into the umbilical arteries which arise as branches of the right and left internal iliacs. At birth, the umbilical vessels contract. Breathing helps to create a negative thoracic pressure thus sucking more blood from the pulmonary artery into the lungs and diverting it from the ductus arteriosis which gradually therefore closes. The foramen ovale is a valvular opening, the valve functioning from right to left side of the heart. The left auricular pressure rises and this valve closes.
The primitive trophoblast erodes the surface of the decidua by enzymatic process, destroys the glands and the stroma and eventually, therefore, penetrates large maternal sinusoids which have formed in the lining of the uterus. The blastocyst now lies in a pool of maternal blood fed by the maternal arterioles and drained by maternal veins. The trophoblast cells proliferate and form pseudopodial like masses which branch repeatedly. This greatly increases the surface area and facilitates foeto-maternal exchange. The trophoblast anchors the blastocyst by adhering to the intervening decidual stroma. The trophoblast differentiates into two layers. The outer or syncitiotrophoblast in contact with the maternal blood becomes the multinucleus syncitium with no distinct cell boundaries, the inner or cytotrophoblast (also called the Langhans layer) forms a single layer of cuboidal cells.
Villi are present over the whole surface of the blastocyst. As this enlarges it compresses the superficial decidua or decidua capsularis and the pregnancy bulges into the uterine cavity. The compression of the decidua capsularis gradually cuts off the circulation through it. This results in atrophy and disappearance of the villi in association with it. The surface of the blastocyst becomes smooth and this portion of the chorion is known as the chorion laeve. At the opposite end of the blastocyst the villi proliferate and enlarge and this is known as the chorion frondosum. The connecting stalk of the embryo is attached to the wall of the blastocyst at this point. Ultimately, with the expansion of the blastocyst, the decidua capsularis comes into contact with the decidua vera and the uterine cavity is obliterated.
The fully formed placenta is a disc approximately two centimeters in thickness tapering towards the edges. It weighs roughly 500 grams and is dark red, the colour being due mainly to the maternal blood within the intervillus spaces. The umbilical cord has two arteries and one vein embedded in Whartons jelly which is a loose myxoematus tissue of mesodermal origin. This jelly acts as a physical buffer and prevents kinking of the cord and interference of maternal-foetal circulation.
The umbilical vessels are generally attached to the placenta near its centre. They immediately divide repeatedly to the form branches which ramify all over the surface. This is known as the "disperse" type of placenta. Occasionally the main vessels may extend almost to the margins of the placenta before dividing (although they give off small branches in their course). This is the "magistral" type of placenta. There is a short communicating branch between the two umbilical arteries just as they reach the placental surface. This serves to equalise the pressure and flow to each half of the placenta.
The functions of the placenta depend on the structure and health of the placental villi. These villi are bathed in maternal blood but there is no direct connection between foetal and maternal blood. There is a foeto-placental barrier. After sixteen to twenty weeks the cytotrophoblast regresses. The syncitiotrophoblast is reduced in thickness as the pregnancy advances. The foetal blood vessels of villi dilate and the mesoderm is reduced in amount. This reduces the physical barrier between foetal and maternal circulations. Indeed, at twelve weeks the barrier is 0.025 millimeters and at term is 0.002 millimeters. The placental functions may be summarized as follows: